The present invention relates to optical imaging and, more particularly, to an apparatus and a method for synchronizing the velocity of an image of a moving object or target and the clocking of image sensor elements used to track the moving target.
A widely used method of electro-optical single-frame imaging involves mechanically sweeping an image past a linear array of photoelements and reading out the array once for each incremental advance of the image. In this way, nearly square frames or long strip-type frames of imagery may be obtained. Certain types of arrays, such as time delay and integrate (TDI) arrays operate the same way, differing primarily in that they collect signals over many line periods.
An early example of utilizing scanned linear imaging arrays is a device having a series of back-to-back silicon diodes in a structure made from two monolithic silicon diode arrays. The array was electronically scanned with the aid of a fixed voltage drop along one of these arrays, together with a voltage sawtooth applied at one terminal. Other types of electronically scanned linear arrays followed, including the use of charge-coupled device (CCD) arrays. The scanning function can be performed in excellent fashion with a simple CCD shift register coupled to the array of photodiodes.
In TDI imaging, each photoelement of the line-scan array is replaced by a light sensing CCD shift register. The TDI imaging CCD arrays are commonly constructed out of a plurality of closely spaced, parallel TDI CCD shift registers built on a photoelectric semiconductor substrate. The array of sensor elements or detectors are used to store an electrical signal representative of the time-integrated radiation intensity. An optical image is scanned across the surface of the photoelectric semiconductor substrate of the TDI imager along the length of the TDI shift registers. The shift registers are clocked in synchronism with the motion of the optical image. Electrons that are freed under one of the TDI shift registers by the photoelectric effect of light in a given portion of the optical image are collected into a charge packet that will be moved along by the TDI shift register in conjunction with that given portion of the optical image. When a charge packet reaches the end of its associated TDI shift register, it is fed into an output CCD shift register in parallel with all of the other charge packets which have reached the end of the their associated TDI shift registers at the same time. The output shift register rapidly shifts out all of the charge packets fed thereto, and provides a series of charge packets or pixel values. The variable charge levels of the series of charge packets correspond to the variable light intensity of a picture line taken from the two-dimensional optical image scanned across the TDI imaging CCD array of the TDI imager.
For a TDI imaging CCD array to function properly without image smearing, the charge packets must be shifted down the shift registers at the same velocity as the image being scanned across the surface of the CCD array.
Embodiments of the present invention are directed to an apparatus and a method for synchronizing the velocity of an image of a moving object or target and the clocking of image sensor elements used to track the moving target. The image of the moving target is projected onto an array of image sensor elements such as a TDI imaging CCD array. The image of the moving target moves in a first direction, and the array of image sensor elements integrate light from the image projected thereon into pixel values, which are shifted along the image sensor elements in the first direction according to a clock rate. The velocity of the image of the moving target is measured by providing a row of image sensors such as a linear CCD imager which extends in the first direction. A portion of the image of the moving target is projected onto the row of image sensors which sense the rate of movement of the portion of the image projected thereon to measure the velocity of the image. The velocity of the image and the clock rate are compared, and the result is used to adjust either the clock rate or the velocity of the target to synchronize the clock rate and the velocity of the image of the moving target.
In accordance with an aspect of the present invention, an imaging apparatus comprises a two-dimensional array of image sensor elements being configured to sense a first set of image elements of a target moving in a first direction with respect to the two-dimensional array of image sensor elements, to integrate light from the set of image elements into corresponding pixel values, and to shift the pixel values along the image sensor elements in the first direction according to a clock rate. A row of image sensor elements extends in the first direction to sense a second set of image elements of the target moving in the, first direction. A synchronization module is coupled with the row of image sensor elements to measure the velocity of the target moving in the first direction based on a rate of movement of the second set of image elements of the target sensed by the row of image sensor elements, and to synchronize the clock rate of shifting the pixel values along the image sensor elements of the two-dimensional array with the velocity of the image elements of the target moving in the first direction.
In some embodiments, the -two-dimensional array of image sensor elements comprises a CCD array, more desirably a TDI imaging CCD array. The row of image sensor elements comprises a linear CCD imager. The TDI imaging CCD array and the linear CCD imager may be formed on a substrate. The synchronization module may comprise a synchronization shift register configured to receive charges generated from the linear CCD imager, and a CCD correlation circuit coupled with the synchronization shift register and configured to correlate signals between different sets of taps positioned along the synchronization shift register to measure the velocity of the image elements of the target moving in the first direction. The CCD correlator generates an output which is directed to a clock timing generator for controlling the clock rate of shifting the pixel values along the image sensor elements of the TDI imaging CCD array to synchronize with the velocity of the image elements of the target moving in the first direction as measured by the CCD correlator. The CCD correlation circuit may be formed on the substrate with the TDI imaging CCD array and the linear CCD imager.
In specific embodiments, the synchronization module is configured to adjust either the velocity of the target moving in the first direction or the clock rate of shifting the pixel values along the image sensor elements of the two-dimensional array in the first direction to synchronize the velocity of the image elements of the target with the clock rate. The two-dimensional array of image sensor elements and the row of image sensor elements are disposed on a focal plane upon which the image elements of the target are projected. An output shift register is disposed to receive the pixel values shifted in the first direction from the two-dimensional array of image sensor elements.
In accordance with another aspect of the present invention, a method for optical imaging comprises projecting a first portion of an image of a target moving in a first direction onto a two-dimensional array of image sensor elements which are configured to integrate light from the image into corresponding pixel values, and shifting the pixel values along the image sensor elements in the first direction according to a clock rate. Each image sensor element of the two-dimensional array continues to integrate light projected thereon into the pixel value shifted therealong. A second portion of the image of the target moving in the first direction is projected onto a row of image sensor elements extending in the first direction. A velocity of the image of the target moving in the first direction is measured based on a rate of movement of the second portion of the image projected onto the row of image sensor elements and sensed by the row of image sensor elements. The method further comprises synchronizing the clock rate of shifting the pixel values along the image sensor elements of the two-dimensional array with the measured velocity of the image of the target moving in the first direction.
In accordance with another aspect of the invention, an imaging apparatus comprises a CCD array of image sensor elements being configured to generate charge packets from a first set of image elements of a target projected on the image sensor elements and moving in a first direction with respect to the CCD array of image sensor elements, and to shift the charge packets along the image sensor elements in the first direction according to a clock rate. A linear CCD imager of image sensor elements extend in the first direction to sense a second set of image elements of the target moving in the first direction. The apparatus further comprises means for measuring a velocity of the image elements of the target moving in the first direction based on a rate of movement of the second set of image elements of the target sensed by the row of image sensor elements. A controller is configured to control either the velocity of the target moving in the first direction or the clock rate of shifting the charge packets along the image sensor elements of the CCD array in the first direction to synchronize the velocity of the image of the target with the clock rate.
In some embodiments, the controller comprises a clock timing generator coupled between the measuring means and at least one clock driver, and being configured to generate a clock rate adjustment based on the measured velocity of the image of the target received from the measuring means and feed the clock rate adjustment to the at least one clock driver to adjust the clock rate of the at least one clock driver used to shift the charge packets along the image sensor elements of the CCD array in the first direction. In other embodiments, the controller comprises a target timing generator coupled between the measuring means and a target driver, and being configured to generate a target velocity adjustment based on the measured velocity of the image of the target received from the measuring means and the clock rate to adjust the velocity of the target moving in the first direction.